[PATCH] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Juergen Gross <jgross@xxxxxxxx>]

The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
potentially with pages being NULL, leading to a NULL dereference.

Fix that by calling unlock_pages only if lock_pages() was at least
partially successful.

Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
Reported-by: Rustam Subkhankulov <subkhankulov@xxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/xen/privcmd.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index 3369734108af..ec87968b4459 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -679,7 +679,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void 
__user *udata)
        rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &pinned);
        if (rc < 0) {
                nr_pages = pinned;
-               goto out;
+               goto unlock;
        }
 
        for (i = 0; i < kdata.num; i++) {
@@ -691,8 +691,9 @@ static long privcmd_ioctl_dm_op(struct file *file, void 
__user *udata)
        rc = HYPERVISOR_dm_op(kdata.dom, kdata.num, xbufs);
        xen_preemptible_hcall_end();
 
-out:
+ unlock:
        unlock_pages(pages, nr_pages);
+ out:
        kfree(xbufs);
        kfree(pages);
        kfree(kbufs);
-- 
2.35.3